It is not enough to have read permission on a file in order to read it. You also need to have read permission on the directory it belongs to. In the case of /sys/kernel/debug/vgaswitcheroo/switch the problem is /sys/kernel/debug which, by default, is readable only by root:

You need to chmod a+r /sys/kernel/debug to have access to the contents of that directory.
(I would not recommend it, since giving global access to /sys/kernel/debug is not a safe thing to do, however this has nothing to do with your question)